Electrophotographic photoreceptors (hereinafter sometimes referred to as photoreceptors) need have a charge retaining function of retaining a charge in a dark place, a charge generating function of generating a charge upon receipt of light, and a charge transporting function of transporting a charge generated upon receipt of light. Such photoreceptors are broadly classified into single-layered type photoreceptors having a layer having all of the above-mentioned functions, and functionally separated multilayer photoreceptors having a charge generation layer which mainly contributes to charge generation and a charge transport layer which contributes to retention of a charge in a dark place and to transport of a charge generated upon receipt of light.
Specific examples of image forming methods using such a photoreceptor include methods using the Carlson process, which typically include the following processes:    (1) charging a photoreceptor in a dark place using corona discharging (charging process);    (2) irradiating the charged photoreceptor with an optical image of an original image such as character images and pictorial images to form an electrostatic latent image on the photoreceptor (latent image forming process);    (3) developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor (developing process);    (4) transferring the toner image onto a recording material (transferring process); and    (5) fixing the toner image on the recording material (fixing process).
After transferring the toner image, the photoreceptor is optionally subjected to other processes such as a discharging process (an electrostatic discharging process or an optical discharging process) of discharging a residual charge on the photoreceptor after transferring the toner image; and a cleaning process of cleaning the surface of the photoreceptor after transferring the toner image, so as to be used for the next image forming processes.
Recently, organic photoreceptors using organic photosensitive materials have been broadly used for electrophotographic image forming apparatuses because of having a good combination of flexibility, thermal stability and film formability. Among such organic photoreceptors, functionally separated multilayer photoreceptors having a photosensitive layer including a charge generation layer including a charge generation material, and a charge transport layer including a charge transport material have been broadly used. Particularly, negatively chargeable photoreceptors having a charge generation layer in which an organic pigment serving as a charge generation material is deposited to form a layer thereof or in which an organic pigment is dispersed in a binder resin, and a charge transport layer in which an organic low molecular weight material serving as a charge transport material is dispersed in a binder resin have been frequently proposed as the functionally separated multilayer photoreceptors.
In addition, recently electrophotographic image forming apparatuses are required to produce full color images and/or to perform high speed image formation so as to have new uses such as small office home office (SOHO) use and printing use as well as office use. In printing use, a greater number of print images are produced per one image forming operation, and there are strict requirements with respect to image qualities. Therefore, the organic photoreceptors used for such electrophotographic image forming apparatuses need have a good combination of durability and electrostatic stability.
In attempting to enhance durability, a technique in which a crosslinked layer is formed as an outermost layer of a photoreceptor is proposed. In this regard, the crosslinked layer is formed by subjecting a material to a three-dimensional crosslinking treatment using energy such as light or electron beams to enhance the abrasion resistance of the photoreceptor. In addition, there are proposals, in which a particulate inorganic or organic material is included in a crosslinked outermost layer, in attempting to further enhance the abrasion resistance. Although the abrasion resistance of a photoreceptor can be enhanced by forming one of these outermost layers thereon, the electrostatic stability of the photoreceptor cannot be fully enhanced.
The reasons why a photoreceptor having a crosslinked outermost layer has an insufficient electrostatic stability is not yet determined, but one of the reasons is considered to be that part of a charge transport material included in the crosslinked material is changed (or decomposed) by the crosslinking energy such as light or electron beams. Specifically, when part of a charge transport material included in the outermost layer is changed by the crosslinking energy, various compounds having different energy levels are present in the outermost layer. In this case, the properties of the photoreceptor are changed after repeated use. For example, decrease of potential of the charged photoreceptor, variation of potential of irradiated portions of the photoreceptor, and decrease of the electric resistance of the surface of the photoreceptor are caused, resulting in deterioration of image qualities such as resolution of images (i.e., formation of blurred images). Since the image qualities are thus deteriorated, the photoreceptor has to be replaced with a fresh photoreceptor, resulting in shortening of the life of the photoreceptor.
Particularly, variation of potential of irradiated portions of the photoreceptor is a serious problem for the image forming apparatuses for use in printing, which are required to have a long life and a high stability. In this regard, variation (job-to-job variation) of potential of irradiated portions of a photoreceptor in a case where the photoreceptor is subjected to one image forming operation, and the next image forming operation is restarted after a pause is more serious than variation (diurnal variation) of potential of irradiated portions of the photoreceptor when the photoreceptor is used for printing images for a relatively long period of time.
Since the diurnal potential variation is not noticeable, and can be corrected in the image forming apparatus, the diurnal potential variation is not a serious problem. By contrast, when a large job-to-job potential variation is caused, change of image qualities of the images is noticeable. Particularly, when the potential of irradiated portions is changed every several or tens of prints, the potential cannot be corrected, thereby causing a serious problem. Particularly, there is a case where a large number of copies of the same image are produced in printing. When a large job-to-job potential variation is caused in such a case, the image density of the copies varies, thereby deteriorating the consistency in image qualities. If the image is a character image, change of the image qualities is hardly noticeable, but when the image is a pictorial full color image, not only the image density but also color tone changes, resulting in occurrence of a serious problem. Therefore, there is a need for a photoreceptor in which the potential of irradiated portions thereof is relatively low while having a low job-to-job variation as well as a low diurnal variation.
In attempting to enhance the electrostatic stability of photoreceptor, various proposals have been made.
For example, there is a proposal for a photoreceptor which has an outermost layer formed by crosslinking a radically polymerizable tri- or more-functional monomer having no charge transport structure and another radically polymerizable monomer having a charge transport structure to impart a good combination of durability and electrostatic stability to the photoreceptor. In addition, another photoreceptor is proposed, in which a polymer obtained by polymerizing and/or crosslinking a compound selected from specific benzidine compounds and a compound selected from specific triphenyl amine compounds is included in the outermost layer to enhance the electric properties of the outermost layer. However, when a charge transport material is polymerized or crosslinked, freedom of the molecules of the material is deteriorated, resulting in deterioration of the charge transport function of the material. In addition, since a reactive charge transport material is used, it is possible that the resultant layer includes an unreacted charge transport material, and/or the charge transport structure thereof is changed when the crosslinking reaction and the polymerization reaction are performed. In this case, the thus degenerated charge transport material is easily affected by acidic gasses, thereby easily causing a problem in that charges are stored in the layer, resulting in deterioration of the electrostatic stability of the photoreceptor.
In addition, in attempting to prevent deterioration of the electric properties of photoreceptor, a photoreceptor is proposed, in which a charge transport polymer is used for a charge transport layer located below a crosslinked outermost layer to prevent migration of the charge transport material to the outermost layer. However, deterioration of the charge transport material included in the outermost layer cannot be prevented when crosslinking the outermost layer using light or electron beams.
Further, another proposal is made, in which an outermost layer is crosslinked using ultraviolet rays with wavelengths of not greater than 310 nm, which can be easily absorbed by organic materials, so that the ultraviolet rays are absorbed only by the surface portion of the layer, in attempting to prevent deterioration of the charge transport material included in the outermost layer. However, it is impossible that the UV crosslinkable charge transport material used for the outermost layer does not absorb the ultraviolet rays, and thereby molecules of the charge transport material are deteriorated, resulting in deterioration of the electrostatic stability of the photoreceptor.
Furthermore, a photoreceptor is proposed, which includes a crosslinked charge transport layer, which is prepared using a radically polymerizable monomer having a charge transport property and which includes a low molecular weight charge transport material, to enhance the electrostatic stability of the photoreceptor. However, when irradiating the charge transport layer with ultraviolet rays to crosslink the layer, the radically polymerizable monomer and the low molecular weight charge transport material are deteriorated, resulting in deterioration of the electrostatic stability of the photoreceptor.
There is another proposal, which includes a photosensitive layer obtained by crosslinking a mixture of a first charge transport material having an acryloyloxy or methacryloyloxy group, and a second charge transport material having a hydroxyl group so that the second charge transport material having a hydroxyl group is incorporated in the three dimensional network of the first charge transport material, and therefore the charge transport groups of the first charge transport material can take an ideal position. However, the second charge transport material having a hydroxyl group has high affinity to moisture, and therefore the photosensitive layer cannot exhibit high stability to withstand environmental conditions. In addition, since two or more charge transport materials having different energy levels are present in the layer, charge transporting cannot be smoothly performed therebetween, thereby deteriorating the electric properties of the layer, resulting in deterioration of the electrostatic stability of the photoreceptor.
Because of these reasons, the inventors recognized that there is a need for a photoreceptor which has a good durability while having little diurnal potential variation and little job-to-job potential variation and which can stably produce high quality images without forming low density images and blurred images.